1. Field of the Invention
The present invention relates to a device for pollution control. More specifically, the present invention relates to device for pollution control where a polluted stream of air or gas is purified from both oxidisable material and nitrogen oxides simultaneously by a combination of regenerative high temperature treatment and catalytic treatment.
2. Description of Related Art
Polluted air and other gases can be purified by heat treatment to such temperatures that the pollutants are oxidised or decomposed. U.S. Pat. No. 4,267,152 and U.S. Pat. No. 4,741,690 describes such processes where the polluted gases are fed through regenerative devices where the heating of a gas is immediately followed by cooling and recovery of the heat content of the gas. In this way heat treatment of the gas to a high temperature can be made in an economical way without a high expenditure of energy.
The incoming raw gas is in these processes successively heated to the high temperature by means of contact with a matrix of solid material capable of heat transfer to the gas. In the solid matrix there is a temperature gradient so that the gas is first successively heated to a maximum temperature. After attaining its maximum temperature the gas is then cooled in an analogous manner by means of contact with a solid matrix of successively lower temperature. In U.S. Pat. No. 4,287,152 the heating and the cooling matrices are separated from each other but used alternatively for heating and cooling purposes according to an alternating direction of gas flow through the matrices. The different matrices are alternatively used for heating and cooling of the gas. In U.S. Pat. No. 4,741,690 there is only one continuous matrix through which the gas flow is being fed. The temperature profile in this matrix is such however that when the gas passes through it is first successively heated to a maximum temperature and then successively cooled.
In both cases the operation is regenerative and the gas is fed in alternating directions through the equipment and is successively first heated to a maximum temperature and then cooled. The maximum temperature employed is such that it is at or above the temperature that is necessary for the intended oxidation or decomposition reaction to take place.
Processes and equipment like these are extensively used for the purification of ventilation air from painting and printing processes. It can also be used for the purification of exhaust gases from internal combustion engines. In this and other cases nitrogen oxides are a part of the pollution problem.
For diesel engines the concentration of nitrogen oxides in the exhaust can reach several thousands of ppm. Good reduction of these nitrogen oxides can be accomplished if the exhaust is first mixed with a corresponding amount of ammonia, urea or other amine compound before it is heat treated in the above described manner. During the heating the mixture will pass through the relevant temperature window for a selective non catalytic reduction (SNCR) reaction where the nitrogen oxides and the amines are transformed to elemental and harmless nitrogen.
Diesel engine exhaust for example can thus be purified in the above described process first during the heating phase when nitrogen oxides are removed and then at the high temperature when soot, aldehydes and other organic matter is oxidised. This way of operation of the equipment is described in European patent number EPC 609 288.
Experience has shown that when the original concentration of nitrogen oxides is more than 1000 ppm a good reduction can be achieved and the concentration nitrogen oxides brought down to a couple of hundred ppm. However when the original concentration already is in this low region as for example in the exhaust from lean burn natural gas engines reduction is very poor.